Method of protecting rotating anode x-ray tubes



April 6, 1943. CASSEN 2,315,593

METHOD OF PROTECTING ROTATING ANODE X RAY TUBES Filed Dec. 4, 1941WITNESSES: v I INVENTOR flamed/c 2 (as: en.

ATTOR Y Patented Apr. 6, 1943 METHOD OF PROTECTING ROTATING AN ODE X-RAYTUBES Benedict Cassen, Pittsburgh, Pa., assignor to WestinghouseElectric &

parry, East Pittsburgh,

Pennsylvania Manufacturing Com-' Pa., a corporation of ApplicationDecember 4, lil il, Serial No. 421,595

9 Claims.

My invention relates to X-ray tubes and, in particular, relates toprotecting the anodes of such tubes from overheating.

In certain types of X-ray tubes, such as are illustrated in the attacheddrawing, the anode comprises a disk which is kept in continuous rotationby an electric motor in order that by continual exposure of a newportion of its surface to the impact of the electrons coming from thecathode, local heating at the point of incidence of the electron streamshall be minimized. Such tubes are adapted to operate with electronstreams of much higher energy concentration than are tubes which employstationary anodes. Even in such tubes, it is impossible, under somedesired conditions of use, to maintain an electron stream in flowcontinuously or for a protracted time, but it suffices for most uses tobe able to radiate the X-rays for a short period. The anode thus absorbsshort period in which the Xrays are turned on, and in the interval ofnon-use following thereafter, the heated portion of the anode is able tocool off substantially. However, after several short-interval exposureshave been taken, separated by only moderate intervals of time, thegeneral anode temperature has been so raised by heat which has not hadtime to be dissipated that further immediate use of the X-rays wouldcause overheating of the anode.

By determining the general surface temperature of the anode, it ispossible to tell whether this dangerous general heating has occurred tosuch an extent as to render immediate use of the tube for X-rayproduction unsafe.

It is, accordingly, one object of my invention to provide means fordetermining the general surface temperature of the portion of the anodeon which electrons from the cathode are incident during operation of thetube.

It is another object of my invention to provide automaticmeans whichwill prevent the use of the tube for X-ray production until the anodetemperature has fallen to such a value as would make immediate operationfor X-ray production safe.

Other objects of my invention will become apparent upon reading thefollowing specification taken in connection with the drawing, in whichthe single figure is a schematic View showing an X-ray tube of therotating anode type in section, together with circuit connectionsadapted to carry out my invention.

Referring to the drawing, conventional rotating-anode an X-ray tube I oftype is provided with energy at a rapid rate during the a cathode 2whichis adapted to be raised to electron-emitting temperature by currentflowing from a transformer 3. The anode 4, that is to say, the memberadapted to act as a target upon which electrons from the cathode 2impinge, is in the form of a disk adapted to be rotated about itscentral axis by an electric motor Within the housing 5. Since therotating anode and electric motor form no part of my invention and arewell known in the art, further detailed description of them is believedto be unnecessary. The anode 4 has a beveled edge 6 which may be facedwith tungsten or other metal suitable to constitute a target for X-rayproduction.

The cathode 2 is preferably housed within a metal enclosure 1 which hasa window 8 through which electrons from the cathode 2 are attracted bythe anode 4 (which is kept strongly positive in potential relative tothe cathode 2 by means not shown), whereby X-rays are produced at anyinstant at the spot on the anode 4 on which the electron beam isincident, the X-rays being radiated through a window 9 in the wall ofthe tube l. The window 8 is of oblong form, its dimension radial of thetube i being preferably approximately three times its circumferentialwidth. The electrons from the cathode 2' thus impinge on a small areaonly of the entire beveled surface of the anode 4. As the anode 4rotates, this area of electron-incidence thus traverses acircumferential path upon its beveled surface, and the development ofheat at the anode surface is thus spread over a wide area instead ofbeing maintained continually concentrated on one spot as in the case ofstationary anode X-ray tubes. This distribution of the heat developmentover a wide area of the surface of the anode i minimizes the temperaturerise at any one point and prevents the destructive heating of thesurface layer of the anode.

While a certain amount of heat is radiated outward from the beveledsurface of the anode 4, considerable quantities likewise flow by thermalconduction inward from the surface into the metallic body of the anodeand accumulate there, being dissipated only slowly from its othersurfaces. If several short but intense exposures to the electron streamare made, the general temperature of the anode 4 rises to such a valuethat further immediate exposure of the anode to electron incidence Wouldcause overheating.

The mass of the cathode filament 2 is relatively small compared withthat of the anode 4, and if current flow to it from the transformer 3 iscut off after an X-ray exposure, it quickly sinks to the vicinity ofroom temperature. However, it is exposed to the radiation of heat fromthe heated anode 2- and the temperature of the cathode quickly attains avalue which is fixed almost entirely by the general temperature of thebeveled surface of anode i. The resistance of tungsten or other metalssuitable for the cathode is a function of their temperature at anyinstant, and I have found that by cutting off heating current from thecathode, permitting it to rest long enough so that its temperature issubstantially determined by that of anode d and then measuring itsresistance, I can determine with suflicient precision for practicalpurposes the temperature at any time of the anode I am thus able todetermine, after several exposures of the anode 4 to electronbombardment of short duration from the cathode 2, whether the generaltemperature of anode i is such as to render use of the X-ray tube forX-ray production safe, or whether it is necessary to wait for a time forthe anode to cool further.

In order to measure the temperature of the cathode 2, I provide theprimary circuit of the transformer 8 with a double-throw switch ii, inone position of which (that shown in full line in the drawing), theprimary of transformer 3 is energized with current from a local source 2for heating the cathode 2 in normal use of the X-ray tube. In the otherposition of the switch i l (shown in dotted line), the primary of thetransformer 3 is connected, through what amounts to analternating-current Wheatstone bridge, to an alternating-current sourceit. The above-mentioned Wheatstone bridge comprises one arm whichcomprises an inductance hi and a resistance E5 in series therewith. Theinductance M and resistance 55 are both variable and the inductance M ispreferably set at a value approximately equal to the inductanceequivalent of the leads as reflected into the primary of the transformer3, its internal leakage inductance, the inductance of the leads from itssecondary to the filament 2, and the inductance of filament 2 itself.The other two arms of the Wheatstone bridge comprise a pair of variableresistors it and l? preferably equal in value. The source it isconnected across one pair of diagonally-opposite corners of theWheatstone bridge, while an alternating-current voltmeter i3 isconnected across the other diagonally-opposite corners of the Wheatstonebridge. The Wheatstone bridge thus described may be used by methods wellknown in the electrical art to measure the temperature of the filament 2and to determine thereby whether, at a suitable interval after movementof the switch H to disconnect the source 112 from transformer 3, it issafe to operate the X-ray tube l.

While I have described the temperature of the cathode filament 2 asbeing measured by means of a Wheatstone bridge arrangement, it will beobvious that any of the other instrumentalities known in the electricalmeasurin art as means for measuring the resistance of an elementpositioned in the secondary of a transformer by readings taken on theprimary circuit of the transformer may be used.

The voltmeter i8 may be provided with a contact it which will be closedupon its needle 2% when the latter stands at the temperaturecorresponding to the maximum value of the resistance of the cathode 2 atwhich it is safe to operate the tube i. The contact it may be connectedthrough a voltage source 2! to the energizing coil 22. of a solenoidarranged upon closure of the contacts 25 and 2G to move the switch IIfrom the position in which it connects the primary of transformer 3 tothe source 53 to the position in which switch i i connects the primaryof transformer 3 to source 52. A spring 24 should then be arranged tobias the switch H to connect the primary of transformer 3 to source it.A latch 25, adapted for manual release should be arranged to releasablyretain the switch of the circuit breaker H in a position in which theprimary of transformer 3 is connected to source E2.

The switch H is maintained in the last-nontioned' position by the latch25 whenever an X-ray exposure is being made and until it is desired tomeasure the resistance of the cathode tube. Whenever the latter desirearises, the latch 25, 25 is released and the spring 24 moves the circuitbreaker ii to connect the primary of transformer 3 to source 83. Thevoltmeter it? then moves to such a position as to indicate theinstantaneous temperature of the cathode 2. The latter temperature will,immediately after employment of the tube i for X-ray production, be sohigh that the needle of galvancmeter 58 will indicate a highertemperature than that corresponding to the position of the contact l9.As the cathode cools, the needle will gradually move until it meets thecontact i9, whereupon the winding 22 will be energized from source 2| tomove the switch it back into the position in which it connects thesource 52 to the primary of transformer 3; and the switch will belatched in this position through closure of the latch 25, 25. Thisposition of the circuit breaker will then persist until the latch 25, 26is manually reopened by the X-ray tube operator to make anothermeasurement of the temperature of cathode 2.

While I have described the instrument i8 as a voltmeter, it will berecognized that it may be replaced by any other well known type of relayoperative to close the circuit of the winding 22 when a predeterminedvalue of voltage exists across the diagonally-opposite terminals of theWheatstone bridge.

While I have described a particular embodiment of my invention, it willbe recognized that its principles are of broader application in wayswhich will be evident to those skilled in the art.

I claim as my invention:

1. In combination with an electrical discharge tube having athermionically-emissive cathode and an anode, said cathode beingpositioned in such proximity to said anode that its temperature isdependent upon the temperature of said anode, a transformer connected todeliver heatmg current for said thermionically-emissive cathode, meansfor connecting. the primary of said transformer to an energy supply andfor connecting the primary of said transformer to an instrumentresponsive to the resistance of the said thermionicaliy-emissivecathode.

2. In combination with an electrical discharge tube having an anode anda thermionicallyemissive cathode positioned in such proximity theretothat its temperature is dependent upon the temperature of said anode,means for measuring the temperature of said. thermionicallyemlssivecathode when it is in a deenergized condition.

3. In combination with an electrical discharge tube having an anode anda thermionicallyemissive cathode positioned in such proximity theretothat its temperature is dependent upon the temperature of said anode,means for measuring the resistance of said cathode when it is in adeenergized condition.

4. The method measuring the resistance 5. The method of measuring thetemperature of the anode of an X-ray tube having athermionicaily-emissive cathode supplied with heat ing current whichconsists in interrupting the flow of heating current and, at asubstantial interval thereafter, measuring the resistance of saidcathode.

6. In combination with an X-ray tube having a rotating anode and acathode positioned in such proximity thereto that its temperature isdependent upon. the temperature of said anode and provided with meansfor supplying it with means for interrupting the flow of heating currentto said cathode, and means for thereafter measuring the resistance ofsaid cathode.

8. In combination with an X-ray tube having a rotating anode and acathode positioned in such proximity thereto that its temperature isdependent upon the temperature of said anode and provided with means forsupplying it with heating current from a transformer of which theprimary is connected to a local energy source, means for interruptingthe flow to said primary of current from said local source and forthereafter measuring through the primary winding thereof the resistanceof the load on said transformer.

9. In combination with an X-ray tube having a rotating anode and acathode positioned in such proximity thereto that its temperature isdependent upon the temperature of said anode and provided with means forsupplying it with heating current from a transformer of which theprimary is connected to a local energy source, means for interruptingthe flow to said primary from said local source, and means forconnecting said primary winding to a relay system adapted toautomatically reinitiate the flow of current to said transformer primaryfrom said local source when the resistance of said cathode reaches apredetermined value.

BENEDICT CASSEN.

